The present invention relates to a method for manufacturing a magnetostrictive shaft, the manufactured magnetostrictive shaft applicable to a magnetostrictive type torque sensor suited for detecting a torque applied to a rotational axis such as an output shaft of an automotive vehicle engine.
In an automotive vehicle on which an automatic power transmission is mounted, a magnetostrictive shaft is attached onto an output shaft of an engine associated with the power transmission to appropriate a gear shift timing of, for example, the automatic power transmission.
FIGS. 1 through 6 show a magnetostrictive torque sensor manufactured by a previously proposed method.
A casing 1 is formed with a stepped cylindrical shape and formed of a non-magnetic material. The casing 1 is fixed onto a casing of the automatic power transmission (not shown).
A magnetostrictive shaft 2 is disposed rotatably on a pair of bearings 3 and 3 fitted within the casing 1.
The magnetostrictive shaft 2 has both ends projected axially from the casing 1 so as to constitute a part of the output shaft, e.g., of the engine.
A shaft parent material 2A of the magnetostrictive shaft 2 is formed in a rod shape and of a structural steel such as Carbon Steel (CS), Nickel-Chromium Steel (SNCM), Nickel-Chromium-Molybdenum Steel Chromium Steel (SCr), Chrome-Molybdenum Steel (SCM), Manganese Steel (SMn), or Manganese-Chromium Steel (SMn C). In addition, a magnetic thin film 2B of the magnetostrictive shaft 2 is provided with a slit forming portion 2C located at a middle portion in the axial direction of the shaft 2. A plurality of slit grooves 4, 4,--are inscribed in line with each other on the part of the outer periphery of the magnetostrictive shaft 2 having obliquely inclined angles of 45 degrees. A plurality of other slit grooves 5, 5,--are inscribed in line with each other on the part of the outer periphery of the magnetostrictive shaft 2 having obliquely inclined angles of 45 degrees.
Each slit groove 4 and each slit groove 5 are spaced apart from each other in the axial direction having a predetermined interval, these slits 4 and 5 being formed at equal intervals over the outer peripheral surface of the slit forming portion 2C. A first magnetic anisotropy portion 6 of the magnetostrictive shaft 2 is formed between each of the slit grooves 4 and a second magnetic anisotropy portion 7 is formed between each of the slit grooves 5. These magnetic anisotropy portions 6 and 7 are formed with magnetic paths caused by a surface magnetic field generated by exciting first and second coils 9 and 10, respectively. The excitation and detection coils 9 and 10 will be described below.
A core member 8 is fixed on an inner peripheral surface of the casing 1 and serves to enclose the slit forming portion 2C of tile magnetostrictive shaft 2 from an outside of the radial direction of the slit forming portion 2C with a space. Each core member 8 is formed in the stepped cylindrical shape by means of the magnetic material such as iron. The excitation and detection coils 9 and 10 are disposed in tile inner side of the core member 8.
The first coil 9 and second coil 10 serve as exciting and detection coils opposed in the radial direction of the shaft 2 against the magnetic anisotropy portions 6 and 7 of the magnetostrictive shaft 2 and disposed in an inner peripheral side of the core member 8 via coil bobbins (not shown). An oscillator 13 to be described later carries out the application of an alternating voltage V to both of the first and second coils 9 and 10 so as enable actions of electromagnetic excitation and magnetic filed change detection, respectively. As shown in FIG. 2, the excitation and detection coils 9 and 10 have self inductances L1 and L2 and iron losses and DC components equivalent to r1 and r2, respectively.
As shown in FIG. 2, a detection circuit 11, namely, an equivalent circuit 11 of the magnetostrictive torque sensor is exemplified. In addition, an oscillator 13 as an AC voltage application source, a differential amplifier 14, and synchronization modulation processing circuit 15 are connected to the detection circuit 11.
The detection circuit 11 includes a bridge circuit 12 having first and second arms of both of the first and second coils 9 and 10 and having third and fourth arms of adjustable resistors R and R. It is noted that a junction a between the first coil 9 and the second coil 10 and another junction b between both of the adjustable resistors R and R are connected across the oscillator 13. The oscillator 13 oscillates and outputs the AC voltage Vf having a frequency of, for example, approximately 30 KHz.
In addition, another junction c in the bridge circuit 12 between the first coil 9 and the one adjustable resistor R is connected to a plus input end of the differential amplifier 14 and another junction d therein between the second coil 10 and the other adjustable resistor R is connected to a minus input end of the differential amplifier 14. Thus, a detection voltage V1 at the junction c and a detection voltage at the junction d are input to corresponding input ends of the differential amplifier 14. An output end 14A of the lo differential amplifier 14 is connected to an input end of the synchronization modulation processing circuit 15. Another input end of the circuit 15 receives the oscillation voltage Vf from the oscillator 13. The synchronization modulation processing circuit 15 synchronizes and rectifies an output voltage E0 from the differential amplifier 14 with the alternating current voltage Vf from the oscillator 13 so as to provide a DC output voltage E (corresponds to a torque detection sensitivity in FIG. 6).
FIGS. 4 and 5 show the previously proposed manufacturing method of the magnetostrictive shaft 2.
First, at a process I, a shaft parent material 2A constituted by the structural steel described above is heated up to a temperature ranging, for example, from about 750.degree. C. about 900.degree. C. to undergo a hardening so as to give the shaft parent material 2A a desired strength.
Next, at a process II, a magnetostrictive material made of such as an iron-aluminium (Fe--Al) alloy is spray coated over a whole periphery of the outer so peripheral surface of the shaft parent material 2A so that the magnetic thin film 2B is integrally formed with the shaft parent material 2A to constitute a shaft material 16.
At a process III, a surface working is carried out for the magnetic thin film 2B covering wholly the outer periphery of the shaft parent material 2A and a mechanical working for each slit groove 4 and 5 is carried out so as to form the magnetic anisotropy portions 6 and 7 on the surface of the magnetic thin film 2B.
At a heating process of process IV, the whole shaft material 16 on which the respective slit grooves 4 and 5 are inscribed is heated under the air or under a nitrogen gas up to, for example, 800.degree. C. to 900.degree. C., preferably about 850.degree. C. This temperature is held for one hour or more.
Next, at a quenching process of process V, the whole shaft material 16 heated at the heating process of process IV is immersed into an oil 17 shown in FIG. 4 so as to perform the oil quenching. In this way, at the heating process IV and quenching process V, a magnetic annealing (heat treatment) is carried out so that a working distortion is eliminated from the shaft material 16 and a hysterisis loop magnitude is made small. Such a processing that deviations of torque detection sensitivity for each of the individual shaft materials 16 and of their hysterisis characteristics are made small is passed to complete the magnetostrictive shaft 2.
A detection operation of the magnetostrictive type torque sensor whose magnetostrictive shaft is manufactured by the previously proposed method described above will be described below.
First, when the AC alternating voltage Vf from the oscillator 13 is applied across the bridge circuit 12 as shown in FIG. 2, magnetic paths are formed along the magnetic anisotropy portions 6 and 7 provided between the respective slit grooves 4 and 5 in the slit forming portion 2C of the magnetostrictive shaft 2. In this case, the adjustable resistors R shown in FIG. 2 is adjusted to provide a zero output voltage E0 from the differential amplifier 14 with a torque applied to the magnetostrictive shaft 2 in zero state.
Then, when a torque is acted upon the magnetostrictive shaft 2 in a direction denoted by T of FIG. 2, a tensile strength +.sigma. is acted along the magnetic anisotropy portion 6 between the respective slit grooves 4 and a compressive stress -.sigma. along the magnetic anisotropy portion 7 provided between the respective slit grooves 5. Therefore, in a case where a positive magnetostrictive material is used in the is magnetostrictive shaft 2, a permeability .mu. of the magnetic anisotropy portion 6 is increased due to the tensile stress +.sigma. and the permeability .nu. is decreased due to the compressive stress -.sigma..
Thus, the self inductance L1 of the excitation and detection coil 9 opposed against the magnetic anisotropy portion 6 of the magnetostrictive shaft 2 is increased on the basis of the increase in the permeability .nu. so that a current flowing through the excitation and detection coil 9 is decreased. On the other hand, the self inductance L2 of the excitation and detection coil 10 opposed against the magnetostrictive shaft 7 is decreased so that a current i2 flowing through the excitation and detection coil 10 is increased.
Consequently, the detection voltage V1 from the excitation and detection coil 9 is decreased so that a detected voltage V2 to be input from the excitation and detection coil 10 to the minus end of the differential amplifier 14 is, in turn, increased. Therefore, since the differential amplifier 14 carries out a differential amplification action such as: E0=A.times.(V1-V2), wherein A denotes an amplification factor of the differential amplifier 14. Then, an AC output voltage EO derived from an output terminal of the differential amplifier 14 is output to the synchronization demodulation processing circuit 15. The synchronization demodulation processing circuit 15 carries out the synchronization demodulation processing for the output voltage E0 in accordance with the AC voltage Vf from the oscillator 13 and rectifies the output voltage E0. As shown in FIG. 6, a torque detection sensitivity (V) is output as a detection signal (as a DC output voltage E) corresponding to the torque (kgf.m) acted upon the magnetostrictive shaft 2.
It is noted that a relationship between the torque (kgf.m) applied to the magnetostrictive shaft 2 and the torque detection sensitivity (V) is expressed in a form of a characteristic line 18 shown in FIG. 6. As a ratio of a maximum output difference .DELTA.Vh to an output full-scale VFS, the hysterisis when the torque is detected is derived as follows: Hysterisis=100.times..DELTA.Vh/VFS (%).
A linearity of the torque detection signal is derived as follows, according to a maximum output deviation .DELTA.Vr with respect to an ideal straight line 19 shown in FIG. 6: EQU Linearity=100.times..DELTA.Vr/VFS (%).
In the previously proposed manufacturing method of the magnetostrictive shaft 2, since the shaft material 16 is heated and oil quenched under the air or nitrogen gas under the heat treatment of the heating process and quenching process, the magnetic thin film 2B is easy to be oxidized or nitrided on the surface of the shaft material 16. This causes the hysterisis to be increased. The linearity becomes worsened, In addition, the torque detection sensitivity (V) is reduced.
On the other hand, if the heating process and oil quenching process are carried out under a vacuum, the surface of the shaft material 16 can be prevented from being oxidized or nitrided. However, in this case, since a cooling velocity of the magnetic thin film 2B becomes slow, the torque detection sensitivity cannot always be improved.
That is to say, as appreciated from Table 1 as will be described later, for example, in a case where an Fe--Al alloy (iron-aluminium alloy) including Aluminum of 15 wt % (weight %), the surface of the magnetic thin film 2B is oxidized under the heating process and the quenching process so that the torque detection sensitivity is reduced up to 0.891 V (volts) with respect to a positive maximum torque and up to -0.921 V with respect to a minus maximum torque.
A Japanese Patent Application First Publication No. Heisei 1-247530 exemplifies a previously proposed heat treatment method of a measured shaft used for the torque sensor. In this Japanese Patent Application First Publication, at least one part of the shaft which constitute the shaft material to be measured comprises Fe--Al alloy (iron-aluminium alloy) having the content of Aluminum ranging from 11 to 14 wt% (weight %). Then, when the part of the shaft is heated and quenched under the vacuum atmosphere, an alloyed state of Fe--Al falls in an .alpha. phase (irregular phase), (.alpha.+B2) phase or (.alpha.+D03) phase, as shown in FIG. 13. It is noted that FIG. 13 will be explained later.
However, when the torque is applied to the shaft manufactured in the above-described method and the shaft is treated so as to increase the torque detection sensitivity, the hysterisis is, in turn, enlarged and the linearity is worsened. On the contrary, when the shaft is treated so as to improve the linearity with the hysterisis reduced, the torque detection sensitivity itself is, in turn, reduced.